The present invention relates generally to an optical system used on imaging apparatus or the like, and more particularly to an imaging optical system used on digital cameras or the like, and an imaging apparatus incorporating the same.
Recently developed cameras using electronic imaging devices, like digital cameras, are being improved in terms of size reductions and performance. With this, improvements in the size and performance of an associated optical system are in mounting need. Lens interchangeable cameras are not exceptional too, because of an increasing demand on their ability to be carried around. A lens system having a wide-angle region represented in terms of the maximum incident half angle of view of 32 degrees or higher (with a focal length of shorter than 35 mm on a 135 format basis) provides an easy-to-use angle of view in view of figure and scenic shots. The following imaging optical systems having a relatively wide-angle region have been known so far in the art.
The lens system disclosed in JP(A) 1-319009 has an exit angle of off-axis rays as large as 20 degrees, although there is a wide angle of view available as represented by a half angle of view of 47 degrees. Here take an interchangeable lens for a telephoto lens as an example. There is an exit angle difference between both lenses generally because the telephoto lens has a small exit angle. For this reason, the use of an electronic imaging device makes a lowering in the quantity of rim rays likely because of ray shading at the respective pixels of the imaging device. Filters such as IR cut filters have their transmittance heavily dependent on the angle of incidence. There is also difficulty in using a common electronic imaging device for both an interchangeable lens type imaging system and a fixed lens type imaging system.
The lens system disclosed in JP(A) 55-67715 has a long back focus, with the whole lens length tending to grow long during image taking.
The lens system disclosed in JP(A) 2007-34103 has an exit angle of the order of 10 degrees, and is less likely to be affected by an electronic imaging device, IR cut filters, etc., but size reductions are hardly achievable because its whole length is long relative to imaging device size.
The lens system disclosed in JP(A) 2001-83411 has a half angle of view of 31.6 degrees or less.
JP(A) 59-216114 discloses a rear focus type lens system. However, the focus group comprises a considerable number of lenses, and there is the need of focusing a lens group that is heavy relative to the size of the lens system. Consequently, it is difficult to obtain stabilized precision upon focus stop.